Exhibit Spotlight: Hesperornis regalis
Since the COVID-19 pandemic has forced many restaurants to temporarily shut down, Hesperornis has resorted to
getting a sashimi fix the old-fashioned way.
What has teeth and couldn't fly? Probably not a bird, unless it happened to be Hesperornis. This Late Cretaceous sea bird retained many features from its dinosaur heritage, and developed some crazy specialized features seen in no other bird lineage, yet among fossil birds its family remains the closest to anatomically modern birds currently known. Nearly 150 years of study and some excellent specimens have given scientists a great look at its anatomy, but it's so weird in so many familiar ways that it remains a tantalizing mystery. Start your own investigation into this prehistoric oddity with the following sampler of Hesperornis factoids!
Hesperornis ranged sea shores from Kansas to Russia and Sweden during the Late Cretaceous, spanning nearly 20 million years. The beaches of Kansas aren't what they used to be: when Hesperornis lived, western Kansas enjoyed a climate not unlike that of modern Florida. Alaska looked more like Oregon, and Arctic ice caps had not yet formed. Birds with a powerful swimming ability would have no problem colonizing both North America and northern Eurasia, no flying necessary. With that kind of geographic and stratigraphic time span, coupled with birds' penchant for diversification, it should come as no surprise that scientists currently recognize 11 species, with more possibly awaiting discovery.
Different depictions of Hesperornis and its kin vary on what to do with its wings. in Hesperornis, they had been reduced to a splint-like humerus. Many argue they would not have appeared outside the animal's body in life, but some depictions still interpret them as tiny stabilizing fins, as seen in the sculptures at the Eccles Dinosaur Park. To date I haven't found any studies providing a detailed case for either representation—personally, I think the former model is more likely, even though I sorta like the look of the fins. Needless to say, such a forelimb skeleton precluded any hope of flight. A few members of the larger hesperornithiform family may have retained flight capability, however.
Most modern diving birds stick to inland waters. Not so Hesperornis—its fossils occur mainly in marine strata. Its leg skeleton provides clues as to why: the specializations involved likely made it a more powerful, more specialized swimmer than any modern diving bird. Comparison with its modern equivalents show that it employed a unique swimming technique that most resembled that of modern cormorants, but not by much. In correlation, some cormorants also inhabit marine environments. The muscle and ligament attachments visible on the bones suggest more powerful swimming strokes than modern birds can replicate. This might have made Hesperornis more capable of coping with oceanic currents. It may also explain the genus' large size for a diving bird.
Some controversy surrounds how to reconstruct its feet. In 1935, Max Stolpe observed similarities between the joints of Hesperornis' toes and those of modern grebes, leading him to propose that Hesperornis had asymmetric lobes on its toes like grebes instead of the webbed feet seen in most specialized swimmers among modern birds. Many workers accepted this model for the next 80 years, some going so far as to say there could be no doubt on the point. However, the most recent comparison of modern and ancient diving birds showed that the features Stolpe used for his argument occur across many species and do not correlate with soft tissues. Without footprints (which might not be possible, considering Hesperornis might not have been capable of walking) or some other kind of fossil impression, the issue may prove difficult to resolve. Considering how grebes' unusual toes correspond to an unusual swimming style that acts like a hydrofoil and incorporates a unique kicking stroke, range of motion studies that test whether Hesperornis could replicate that grebe kick could provide valuable insights on the question.
The sculptures at the Dinosaur Park have their share of anatomic mistakes. Some are forgivable: though Hesperornis probably didn't perch on rocks as shown here, such a pose does make them more visible than placing them underwater. One problem that we can't fix on the current sculptures, though, is those knees. Not only would the thighs have remained buried in the body cavity of the animal, its shins probably did so as well. Anatomical limitations like these of course explain why I said they probably didn't perch on rocks as shown.
While Hesperornis' knees likely didn't jut out of its body, it did have pointy kneecaps. These served as muscle attachments for the powerful kicking muscles in the lower legs and feet.
Though many parts of Hesperornis anatomy provokes interesting questions, most studies focus on the legs and the head. Having already discussed its legs, let's look at the head. Like other piscivores ranging from mosasaurs to pelicans, Hesperornis featured a joint in the middle of its jaws that allowed for a hydrodynamic head when the jaw was closed, but upon opening expanded the gape for swallowing large fish. However, Hesperornis and its closest relatives used a strategy different from other reptiles and birds. The upper jaw also seems to have featured a kinetic design, but exactly how it worked has generated considerable controversy. Some scientists maintain that Hesperornis skulls featured the same kinetic joints as modern birds. Others have proposed a more novel system of head hinges, including bending at the maxilla in a way unknown among vertebrates. The most recent attempt to explain its weird skull was reported to the Society of Vertebrate Paleontology during the 2016 annual meeting in Salt Lake. That team of researchers collected CT scans of Hesperornis skull bones and assembled a virtual 3D version calculated to compensate for distortions arising from fossilization. They concluded that Hesperornis could effect a skull bend similar to that of herons, something declared impossible by earlier studies, but that it accomplished the motion in a unique way by using part of the beak as a spring. Considering the unique hinge in the lower jaws this might not come as a surprise, but the issue probably requires further exploration.
One feature that electrified observers when scientists first revealed Hesperornis was its pointy teeth. The lower jaw included them throughout the length of the dentary. The upper jaw only included teeth in the rear half of the beak. These teeth were small but sharp and recurved, great for holding fish. They differ from classic dinosaur teeth in several respects. First, they feature only one simple layer of enamel, suggesting that the process of eliminating the teeth in birds was already well underway. Even though they developed in sockets like other members of archosauria, they had lost the periodontal ligament that holds the teeth in the jaw. To compensate, they developed a unique groove where the teeth could be cemented into the jaw with a thin layer of cementum. Since our Hesperornis models are posed with closed mouths, you won't see their teeth.
Many visitors to the park ask about the monochrome color scheme of our Hesperornis sculptures or assume they represent primitive penguins. We chose this color scheme for several reasons. First, recent studies have shown that monochrome works well for all sorts of marine animals, especially ones belonging to terrestrial lineages that "returned to the sea." Puffins, gulls, auks, penguins, leatherbacks, orcas, narwhals, dolphins . . . the modern examples abound, but mosasaur and ichthyosaur skin fossils likewise preserve evidence that they followed suit. Black and white is and was a maritime color scheme, and Hesperornis was a maritime animal. Second, we may have direct evidence of color in a close Hesperornis relative. Dark pigmentation in animals tends to come from a chemical called eumelanin, and recent studies suggest eumelanin can act like a preservative for skin elements if the conditions are right. If so, dark skin structures are more likely to survive as fossils than light colored elements. In 1896 S.W. Williston reported on a Hesperornis fossil that preserved feathers and leg scales. In depth studies on this specimen have not examined the mode of preservation at work, but if it records certain chemical reactions instead of just impressions, it could support a dark grey or black color scheme for parts of this animal. Well, one of its close relatives, anyway: nearly 100 years later, scientists referred this specimen to a new genus, Parahesperornis. Just when you think you've got a new angle on a question, something always comes up!
Zinoviev, A. V. (2009). CRETACIOUS FOOT-PROPELLED BIRD HESPERORNIS: ITS OWN WAY. In Materialien zum wissenschaftlichen Seminar der Stipendiaten der Programme «Michail Lomonosov II» und «Immanuel Kant II (Vol. 2010, pp. 212-216).
Dumont, M., Tafforeau, P., Bertin, T., Bhullar, B. A., Field, D., Schulp, A., ... & Louchart, A. (2016). Synchrotron imaging of dentition provides insights into the biology of Hesperornis and Ichthyornis, the “last” toothed birds. BMC evolutionary biology, 16(1), 178.
Lindgren, J., Sjövall, P., Carney, R. M., Uvdal, P., Gren, J. A., Dyke, G., ... & Polcyn, M. J. (2014). Skin pigmentation provides evidence of convergent melanism in extinct marine reptiles. Nature, 506(7489), 484-488.